faster implementation of sum ( for Codility test )

Question

How can the following simple implementation of sum be faster?

private long sum( int [] a, int begin, int end ) {
    if( a == null   ) {
        ret         


        

Answer 1

Here is my solution and I scored 100%

 public static int solution(int[] A)
    {
        double sum = A.Sum(d => (double)d);
        double leftSum=0;
        for (int i = 0; i < A.Length; i++){
            if (leftSum == (sum-leftSum-A[i])) {
                return i;
            }
            else {
                leftSum = leftSum + A[i];
            }
        }
        return -1;
    }